Semiconductor package with getter formed over an irregular structure

ABSTRACT

A semiconductor package includes a substrate having a first surface portion in a cavity. The first surface portion includes an artificially formed grass structure. The package includes a getter film formed over the grass structure.

BACKGROUND

The ability to maintain a low pressure or vacuum for a prolonged periodin a microelectronic package is increasingly being sought in suchdiverse areas as field emission displays (FEDs),micro-electro-mechanical systems (MEMS), and atomic resolution storagedevices (ARS). For example, computers, displays, and personal digitalassistants may all incorporate such devices. Both FEDs and ARS devicestypically have two surfaces juxtaposed to one another across a narrowvacuum gap. Typically electrons traverse this gap either to excite aphosphor in the case of FEDs or to modify a media in the case of ARSdevices.

One of the major problems with vacuum packaging of electronic devices isthe continuous outgassing of hydrogen, water vapor, carbon monoxide, andother components of the electronic device. To minimize the effects ofoutgassing, gas-absorbing materials commonly referred to as gettermaterials are typically used. Typically, a separate cartridge, ribbon,or pill that incorporates the getter material is inserted into theelectronic vacuum package.

In conventional getter cartridges, the getter material is deposited ontoa metal substrate and then activated using electrical resistance, RF, orlaser power to heat the getter material to a temperature at which thepassivation layer on the surface diffuses into the bulk of the material.Non-evaporable getter material is activated in a temperature range of250°-900° C., depending on the particular material used.

Getter materials have also been deposited on flat surfaces within vacuumpackages. A problem with this approach is that the surface area withinthe vacuum package that is available for getter deposition is typicallylimited. At the wafer level, there is a competition between activedevice area and the area available for getter. To achieve a good vacuumin a sealed package and maintain a low pressure over the lifetime of thedevice, a high fraction of the surface area inside the package should begetter. Providing a sufficient amount of getter material within thevacuum package is difficult when there is a limited amount of surfacearea available for getter deposition.

In some conventional vacuum packages, the surface area for getterdeposition has been increased by forming an array of columns in a flatsurface within the vacuum package. Photolithography techniques are usedto define the array of columns. A problem with this approach is that theuse of photolithography to define an array of columns increases thecomplexity and cost of making the package.

SUMMARY

One form of the present invention provides a semiconductor package. Thesemiconductor package includes a substrate having a first surfaceportion in a cavity. The first surface portion includes an artificiallyformed grass structure. The package includes a getter film formed overthe grass structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is diagram illustrating a cross-sectional view of a substratebefore it is processed according to one embodiment of the presentinvention.

FIG. 2 is a diagram illustrating a cross-sectional view of the substrateshown in FIG. 1 with a grass structure formed on a top surface of thesubstrate according to one embodiment of the present invention.

FIG. 3 is a scanning electron microscope (SEM) image illustrating aperspective view of a semiconductor substrate with a grass structureformed on a top surface of the substrate according to one embodiment ofthe present invention.

FIG. 4A is diagram illustrating a cross-sectional view of the substrateshown in FIG. 2 with a layer of getter formed on the grass structureaccording to one embodiment of the present invention.

FIG. 4B is a diagram illustrating a cross-sectional view of thesubstrate shown in FIG. 2 with an adhesion layer formed on the grassstructure and a getter layer formed on the adhesion layer according toone embodiment of the present invention.

FIG. 5 is an SEM image illustrating a perspective view of asemiconductor substrate with a getter covered grass structure formed ona top surface of the substrate according to one embodiment of thepresent invention.

FIG. 6 is an SEM image illustrating a perspective view of asemiconductor substrate with a getter covered grass structure formed ona top surface of the substrate according to another embodiment of thepresent invention.

FIG. 7 is a flow diagram illustrating a method for processing asubstrate according to one embodiment of the present invention.

FIG. 8 is diagram illustrating a cross-sectional view of ahermetically-sealed microelectronic package with a plurality of gettercovered grass structures according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” etc., is used with reference to theorientation of the Figure(s) being described. Because components ofembodiments of the present invention can be positioned in a number ofdifferent orientations, the directional terminology is used for purposesof illustration and is in no way limiting. It is to be understood thatother embodiments may be utilized and structural or logical changes maybe made without departing from the scope of the present invention. Thefollowing Detailed Description, therefore, is not to be taken in alimiting sense, and the scope of the present invention is defined by theappended claims.

FIG. 1 is a diagram illustrating a cross-sectional view of a substrate100 before it is processed according to one embodiment of the presentinvention. In one embodiment, substrate 100 is a semiconductorsubstrate, such as a silicon wafer or a portion of a silicon wafer. Inanother embodiment, substrate 100 is a glass substrate (e.g., silicondioxide or fused silica). Substrate 100 includes a top surface 102 and abottom surface 104, which are both substantially flat surfaces. In oneform of the invention, substrate 100 is processed to create a grassstructure on either the top surface 102 or the bottom surface 104, orboth the top surface 102 and the bottom surface 104, with the grassstructure providing an increased surface area for deposition of a gettermaterial, as described in further detail below with reference to FIGS.2-8.

FIG. 2 is a diagram illustrating a cross-sectional view of the substrate100 shown in FIG. 1 with a “grass” structure 202 formed on a top surfaceof the substrate 100 according to one embodiment of the presentinvention. Structure 202 is referred to herein as a “grass” structure ora “silicon grass” structure because, in one embodiment, the structure202 is formed in a silicon wafer, and has a random and irregularappearance that resembles lawn grass. In one embodiment, grass structure202 is artificially formed in substrate 100 by performing a reactive ionetch on the top surface 102 (FIG. 1) of the substrate 100.

As shown in FIG. 2, grass structure 202 has an irregular and randomarrangement of features or protruding members 204 with varyingdimensions and with valleys 206 between the features 204. In oneembodiment, the features 204 of grass structure 202 have a rough,substantially conical appearance, and closely resemble a plurality ofstalactites or stalagmites. In one form of the invention, the features204 of grass structure 202 have a mean height of about 1.0 to 3.0microns, a mean width of about 0.25 to 0.75 microns, and the meanspacing between features 204 is about 0.5 to 1.0 microns.

FIG. 3 is a scanning electron microscope (SEM) image illustrating aperspective view of a semiconductor substrate with a grass structure 202formed on a top surface of the substrate according to one embodiment ofthe present invention. A scale of 500 nanometers (nm) is shown at thebottom of the SEM image shown in FIG. 3. In one embodiment, the grassstructure 202 is formed across the entire top surface 102 of substrate100 (FIG. 1) without using photolithography. In another embodiment, aphotomask or shield is used to define the boundaries of a region ormultiple regions of the substrate 100 on which the grass structure 202is to be formed, with each such region including a plurality ofstalactite-shaped features 204 (FIG. 2). In contrast, prior techniquesthat involve depositing getter on a regular array of columns use aphotomask to separately define each of the individual columns in thearray.

FIG. 4A is a diagram illustrating a cross-sectional view of thesubstrate 100 shown in FIG. 2 with a layer or film of getter 402 formedon the grass structure 202 according to one embodiment of the presentinvention. In one form of the invention, the getter layer 402 isdeposited on the grass structure 202 by sputter deposition. In oneembodiment, getter layer 402 is fabricated using a non-evaporable gettermaterial. In other embodiments, an evaporable getter material may beused. Getter materials include titanium, zirconium, thorium, hafnium,vanadium, yttrium, niobium, tantalum, and molybdenum. Preferably, getterlayer 402 is a zirconium-based alloy such as zirconium-aluminum,zirconium-vanadium, zirconium-vanadium-titanium, orzirconium-vanadium-iron alloys, and more preferablyzirconium-vanadium-titanium, or zirconium-vanadium-iron alloys, becauseof the lower activation temperatures used for these materials. In oneembodiment, getter layer 402 has a uniform thickness of between about0.1 to 2 microns. In another embodiment, getter layer 402 has a uniformthickness of between about 0.25 microns to 0.75 microns.

Incorporation of a getter covered grass structure, such as shown in FIG.4A, into a vacuum cavity of a semiconductor package helps to maintain avacuum in the cavity. The getter layer 402 forms a “pump” where the areaand volume of getter material 402 determines the capacity of the pump.Normally, there is a passivation layer on the surface of getter material402 when exposed to ambient conditions. However, when heated tosufficiently high temperatures, the passivation layer diffuses into thebulk of getter layer 402, resulting in activation of getter layer 402.This process of activation forms a clean surface upon which additionalmaterial may adsorb. In one embodiment, getter layer 402 is activated byheating the semiconductor package in an oven. In other embodiments,radio frequency (RF), laser power, or other heat sources may be used toactivate getter layer 402. The actual temperature used for activationdepends on the particular composition of getter layer 402 and ispreferably in the range of about 250° to about 450° C.

FIG. 4B is a diagram illustrating a cross-sectional view of thesubstrate 100 shown in FIG. 2 with an adhesion layer 404 formed on thegrass structure 202 and a getter layer 402 formed on the adhesion layer404 according to one embodiment of the present invention. In theembodiment shown in FIG. 4A, the getter layer 402 is deposited directlyon the grass structure 202. In the embodiment shown in FIG. 4B, anadhesion layer 404 is deposited on the grass structure 202, and then thegetter layer 402 is deposited on the adhesion layer 404. The adhesionlayer 404 is used in one embodiment to improve the adhesion between thegetter layer 402 and the grass structure 202. In one form of theinvention, the adhesion layer 404 and the getter layer 402 are bothdeposited by sputter deposition. In one embodiment, the adhesion layer404 is tantalum (Ta), and has a thickness of about 300 angstroms. Inanother embodiment, the adhesion layer 404 is Ti or TiW. In yet anotherembodiment, the adhesion layer 404 is made from any of the gettermaterials listed above, or another material having adhesive properties.In one form of the invention, the adhesion layer 404 has a thickness inthe range of 100 to 1000 angstroms.

FIG. 5 is an SEM image illustrating a perspective view of asemiconductor substrate with a getter covered grass structure 502 formedon a top surface of the substrate according to one embodiment of thepresent invention. A scale of 2 micrometers (μm) is shown at the bottomof the SEM image shown in FIG. 5. The getter layer in the embodimentshown in FIG. 5 is relatively thin (e.g., about 0.1 microns) andprovides a conformal coating of the grass structure, so the gettercovered grass structure 502 has substantially the same shape as thegrass structure itself. Compared to a conventional flat surface for thedeposition of a getter material, the grass structure 202 (FIG. 2)greatly increases the available surface area on which the gettermaterial is deposited. A greater height of the features 204 of the grassstructure 202 and a greater number of such features 204 increase thetotal getter surface area.

FIG. 6 is an SEM image illustrating a perspective view of asemiconductor substrate with a getter covered grass structure 602 formedon a top surface of the substrate according to one embodiment of thepresent invention. A scale of 2 micrometers (μm) is shown at the bottomof the SEM image shown in FIG. 6. The getter layer in the embodimentshown in FIG. 6 is thicker than the getter layer in the embodiment shownin FIG. 5, and the resulting getter covered grass structure 602 haspeaks that are more rounded than those shown in FIG. 5. As the thicknessof the getter layer increases, the getter covered features of the grassstructure become larger and more rounded, and the overall surface areaof the getter layer decreases. In one embodiment, the getter layer 402(FIGS. 4A and 4B) is deposited at a thickness that has a high surfacearea for gettering most gases but a sufficient volume for absorbinghydrogen.

FIG. 7 is a flow diagram illustrating a method 700 for processing asubstrate according to one embodiment of the present invention. At 702,a grass structure 202 (FIG. 2) is formed in a surface of a substrate100. In one embodiment, the grass structure 202 is formed by a reactiveion etch process. In one form of the invention, the grass structure 202is formed with an advanced silicon etch process using a conventional dryetch tool with an inductively coupled plasma system. In one embodiment,the etch process involves a periodic etch, polymer deposition, etch,polymer deposition, etc., process, with an etch cycle of nine seconds, apassivation cycle of nine seconds, and a total etch time of ten minutes.A goal in the etch process is to create an imbalance of higherpassivation than etch performance, thereby creating inefficient etchconditions. This condition causes micromasking at the etch surface,which causes a reduced etch rate and creates etch tunneling and grasstype features 204 (FIG. 2) to be etched at the surface of the substrate100.

In another form of the invention, the grass structure 202 is formed at702 by depositing a thin metal layer on the substrate 100 to serve as amask, annealing the metal layer to cause the metal layer to agglomerateinto islands of the desired size and spacing, and then using this metallayer as an etch mask for the underlying substrate 100 during the etchprocess. In one embodiment, the metal layer is gold deposited byevaporation or sputter deposition, with a thickness in the range of20-100 angstroms. In one form of the invention, the annealing of thegold layer is done in a rapid anneal system for a time in the range of10 seconds to 10 minutes, and at a temperature in the range of 300°-700°C. In one embodiment, the etch process continues until the thin metalmask layer is completely removed so that the metal layer does notinterfere with the getter film chemistry.

At 704, a sputter etch is performed on the grass structure 202 to removethe native oxide on the grass structure 202. At 706, an adhesion layer404 is deposited on the grass structure 202. In one embodiment, theadhesion layer 404 is deposited by sputter deposition. At 708, a getterlayer 402 or 602 is deposited on the adhesion layer 404. In oneembodiment, the getter layer 402 or 602 is deposited by sputterdeposition. In another embodiment of the present invention, an adhesionlayer 404 is not used, and the getter layer 402 or 602 is depositeddirectly on the grass structure 202 as shown in FIG. 4A.

In another form of the invention, at 708 in method 700, the getter layer402 or 602 is an evaporable getter material, which is deposited on theadhesion layer 404 (or directly on the grass structure 202) byevaporation. In one embodiment, the evaporable getter material is barium(Ba). In one form of the invention, at step 708, the method 700 uses athermite reaction between BaAl₄ and Ni to release Ba inside a sealedpackage, and thereby form the barium getter layer 402 or 602. In anotherform of the invention, at 708 in method 700, the BaAl₄ and Ni mixture isdeposited as alternating layers inside a package, and a laser is used toheat the layers and initiate the reaction. The reaction causes the Ba tobe released in the package and form the barium getter layer 402 or 602.

FIG. 8 is diagram illustrating a cross-sectional view of ahermetically-sealed microelectronic package or device 800 with aplurality of getter covered grass structures according to one embodimentof the present invention. Microelectronic package 800 is also referredto herein as semiconductor package 800. Microelectronic package 800includes top substrate 802 and bottom substrate 816, which are connectedtogether via seal ring 812. A vacuum cavity 814 is defined within thepackage 800 by the substrates 802 and 816, and the seal ring 812. In oneembodiment, vacuum cavity 814 is maintained at a pressure of less than10⁻³ torr. In another embodiment, vacuum cavity 814 is maintained at apressure of less than 10⁻⁵ or 10⁻⁶ torr. In one form of the invention,substrates 802 and 816 are semiconductor substrates. In one embodiment,substrates 802 and 816 are silicon die. In another embodiment, at leastone of the substrates 802 and 816 is a glass substrate. In one form ofthe invention, substrate 802 is a semiconductor substrate, and substrate816 is a glass substrate. Top substrate 802 includes an active region808 formed on a bottom surface of the substrate 802. Bottom substrate816 includes an active region 810 formed on a top surface of thesubstrate 816. In one embodiment, at least one semiconductor device isformed in each of the active regions 808 and 810.

Grass structures 804A and 804B are formed in the bottom surface of thesubstrate 802 adjacent to the active region 808. Grass structures 804Cand 804D are formed in the top surface of the substrate 816 adjacent tothe active region 810. Getter layers 806A-806D (collectively referred toas getter layers 806) are formed over grass structures 804A-804D(collectively referred to as grass structures 804), respectively. Inanother embodiment of the present invention, grass structures 804 andgetter layers 806 are formed on one of the substrates 802 or 816, ratherthan both of the substrates 802 and 816. In one embodiment, grassstructures 804 are formed by a reactive ion etch process, and getterlayers 806 are deposited by a sputter deposition process. In oneembodiment, during sealing of the microelectronic package 800, thepackage 800 is heated to activate the getter layers 806. The actualtemperature used for activation depends on the particular composition ofthe getter layers 806 and is preferably in the range of about 250° toabout 450° C.

By creating grass structures 804 in the surfaces of substrates 802 and816, the available surface area for depositing getter within vacuumcavity 814 is greatly increased. In one embodiment, the grass structures804 are formed without using lithographic patterning to define eachindividual feature of the grass structures 804.

In one form of the invention, substrates 802 and 816 are semiconductordie that include semiconductor devices for providing electronicfunctionality, and substrates 802 and 816 also form part of the packageitself. In one embodiment, microelectronic package 800 is an atomicresolution storage (ARS) device or a field emission display (FED) devicewith an electron emitter for emitting electrons across the vacuum cavity814. The emitted electrons traverse the vacuum cavity 814 to excite aphosphor in the case of a FED device or to modify a media in the case ofan ARS device. In another embodiment, microelectronic package 800 is amicro-electro-mechanical system (MEMS) device.

One form of the present invention provides a grass structure to increasethe surface area of a getter material and thereby increase the gettercapacity and improve the efficiency of removing gas molecules in avacuum-sealed electronic package. To achieve a very low pressure vacuum(e.g., pressures of 10⁻⁵ or 10⁻⁶ torr or less), a large getter surfacearea is desired. By depositing the getter on etched silicon grassaccording to one embodiment, it is possible to increase the gettersurface area and thereby improve the ultimate vacuum of the package.Embodiments of the present invention are particularly useful inmicroelectronic or micro-electro-mechanical systems where an ultra-highvacuum is needed and silicon area is limited.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. A semiconductor package, comprising: a substrate having a first surface portion in an enclosed cavity, the first surface portion including an artificially formed grass structure, wherein the grass structure comprises an irregular arrangement of protruding members; and a getter film formed over the grass structure.
 2. (canceled)
 3. The semiconductor package of claim 1, wherein the protruding members are one of substantially stalactite-shaped and substantially conically-shaped.
 4. The semiconductor package of claim 1, wherein the protruding members having varying dimensions.
 5. The semiconductor package of claim 1, wherein the grass structure is formed by reactive ion etching.
 6. The semiconductor package of claim 1, wherein the getter film is formed over the grass structure by sputter deposition.
 7. The semiconductor package of claim 1, wherein the getter film comprises a non-evaporable getter material.
 8. The semiconductor package of claim 7, wherein the non-evaporable getter material comprises a metal selected from the group consisting of molybdenum, titanium, thorium, hafnium, zirconium, vanadium, yttrium, niobium, tantalum, and combinations thereof.
 9. The semiconductor package of claim 7, wherein the non-evaporable getter material is a zirconium based alloy.
 10. The semiconductor package of claim 9, wherein the non-evaporable getter material is one of a zirconium-vanadium-titanium alloy and a zirconium-vanadium-iron alloy.
 11. The semiconductor package of claim 1, wherein the getter film comprises an evaporable getter material.
 12. The semiconductor package of claim 11, wherein the evaporable getter material is barium.
 13. The semiconductor package of claim 1, and further comprising an adhesion layer formed on the grass structure, and wherein the getter film is formed on the adhesion layer.
 14. The semiconductor package of claim 13, wherein the adhesion layer is one of Ta, Ti, and TiW.
 15. The semiconductor package of claim 1, wherein the grass structure is formed without using lithographic techniques to define individual features of the grass structure.
 16. The semiconductor package of claim 1, wherein the substrate is a semiconductor substrate.
 17. The semiconductor package of claim 16, wherein the semiconductor substrate is a silicon wafer.
 18. The semiconductor package of claim 1, wherein the substrate is a glass substrate.
 19. The semiconductor package of claim 1, wherein the semiconductor package is one of an atomic resolution storage (ARS) device, a field emission display (FED) device, and a micro-electro-mechanical systems (MEMS) device.
 20. The semiconductor package of claim 1, wherein the cavity is a low-pressure cavity.
 21. The semiconductor package of claim 1, wherein the cavity is a vacuum cavity.
 22. The semiconductor package of claim 1, wherein the grass structure is formed by depositing a metal layer over the substrate, annealing the metal layer, and etching the metal layer and the substrate. 23.-45. (canceled)
 46. A microelectronic package, comprising: a semiconductor substrate having at least one electronic device positioned within a cavity of the package; and means for maintaining a low pressure within the cavity.
 47. The microelectronic package of claim 46, wherein the means for maintaining a low pressure within the cavity comprises an irregular arrangement of features artificially formed in a surface of the semiconductor substrate, and a getter film formed over the irregular arrangement of features.
 48. The microelectronic package of claim 46, wherein the low pressure is a vacuum pressure.
 49. The microelectronic package of claim 46, wherein the low pressure is less than about 1 torr.
 50. The microelectronic package of claim 46, wherein the low pressure is less than about 10⁻⁵ torr.
 51. The microelectronic package of claim 46, wherein the low pressure is less than about 10⁻⁶ torr.
 52. A semiconductor package, comprising: a substrate having a first surface portion in a cavity, the first surface portion including an artificially formed grass structure; a getter film formed over the grass structure; and an adhesion layer formed on the grass structure, and wherein the getter film is formed on the adhesion layer. 